The present invention relates to computer systems and in particular to an improved technique for displaying decoded television video images on a computer monitor as a window in a computer desktop display.
A typical computer system for example, a conventional Personal Computer (PC) may be augmented with a television (TV) receiver card capable of being plugged in to a Peripheral Component Interchange (PCI) bus of that PC. The card enables the PC to receive the full range of television broadcast including digital and analog television signals. A card of this type normally has a coaxial connector for an antenna input. While one is watching TV, the Central Processor Unit (CPU) of the PC is not involved in the process. Thus, the television image can be resized so that, for example, the viewer can check his or her e-mail without affecting the television image.
In a typical multimedia computer system, displaying decoded television video signals from an analog TV card is typically performed by decoding and digitizing the analog video signal and sending digital video signals over the PCI bus or other computer bus to a graphics card. The digital video signals are then displayed on a computer monitor as a window on the computer desktop display. Such devices have been unsatisfactory in that only low-resolution video can be displayed due to limitations resulting from the bandwidth of the computer bus.
The Graphics card, which is also plugged into the PCI bus, contains the necessary video memory and other electronics to provide a bitmap display and normally requires a software driver such as Microsoft Direct Draw ®.
As used herein, the terms “TV Card” and “Graphics Card” are not restricted to separate printed circuit cards that may be plugged into or removed from connectors mounted on the PC motherboard. These terms also include TV processors and graphics processors or corresponding software processes that are implemented in other forms, such as being a part of the motherboard or that are implemented in computer systems that do not include a motherboard.
TV cards are incapable of providing a smooth or stable picture when used in combination with a graphics card. The resulting image may exhibit, for example, a slight jittery up and down motion that is very distracting. Other problems with existing TV cards include digital artifacts such as jagged borders between light and dark areas. Other types of distortion include signal converter interference which may produce wavy lines running at vertical angles through the picture.
FIG. 1 illustrates a conventional circuit within a personal computer 120. The TV card 130 receives broadcast television signals from an antenna 100 and outputs a digitized video signal to a PCI bus 140 for transmission along the bus. A graphics card 150 connected to the PCI bus 140 receives the digitized video signal and after processing, outputs a RGB video signal 162 to a computer monitor 170. The decoded TV signal is represented on the computer monitor 170 as an area 172 in the computer desktop display 174. This method has a serious drawback because it can only display low resolution video due to the limitation on the PC bus bandwidth.
Again referring to FIG. 1, another method to input television video into the desktop display uses a feature connector. The feature connector is a 8 bit expansion bus for a graphics card that has been standardized by Video Electronic Standards Association (VESA). The feature connector is provided on some graphics cards to receive video images from a TV card to avoid the bandwidth limitations of the PCI bus. Transferring video over a cable 134 from the TV card 130 to the graphics card 150 creates potential for violating the copyright protection of the digital video, however, because a peripheral taping device could easily be connected to the cable to intercept the signal while transferring the digital video information.
Another method for inserting television video into a desktop display is disclosed in U.S. Pat. No. 5,621,428 entitled AUTOMATIC ALIGNMENT OF VIDEO WINDOW ON A MULTIMEDIA SCREEN, to King et al. This patent describes a method by which television video data is merged into a desktop display using a conventional chroma key technique. The video card inserts a predetermined color into a window area on the desktop display where the TV video data will be displayed. An analog multiplexer then switches the television video data into the desktop display to replace pixels having the predetermined color.
One problem with this system is the alignment of the television signal and the window in the desktop display. The referenced patent overcomes this problem by storing a predetermined color as a color key in place of the inset image in a motion video memory. The system detects the chroma color and the color key value in the combined image produced by the graphics card to determine, by their relative timing, if the inset image is not properly aligned with the desktop display. Any misalignment is compensated for by adjusting the delay in providing the television video data from the buffer memory. Once this timing has been adjusted, the color key data is replaced by active video.